1. Field of the Invention
The present invention relates to the field of fabrication of semiconductor devices. More specifically, the invention relates to forming bipolar transistors.
2. Background Art
Increasing demand for higher-performing analog circuits, such as high speed, low noise amplifiers, provides an impetus for continued improvements in complementary bipolar technology. In complementary bipolar technology, PNP and NPN devices can be integrated on the same substrate of a semiconductor die in a complementary bipolar process. However, semiconductor manufacturers are challenged to provide a complementary bipolar process that effectively integrates higher performing PNP and NPN devices.
In one approach, a lateral PNP device has been integrated with a silicon-germanium (“SiGe”) NPN device in a complementary bipolar process. However, in a lateral PNP implementation, the base width of the PNP device, which is defined by two mask edges, is difficult to control, which negatively affects the performance of the lateral PNP device. In contrast to a lateral PNP device, a vertical PNP device has a base width that is defined by base implant depth, which is easier to control than the alignment of the mask edges utilized to define base width in the lateral PNP device. As a result, the base in the vertical PNP device can be made narrower than the base in the lateral PNP device, which enables a vertical PNP device to have a larger current gain than a lateral PNP device. For example, a vertical PNP device can achieve a current gain of between approximately 50 and approximately 100, while a lateral PNP device can only achieve a current gain of approximately 15.
Thus, in order to achieve increased performance, it is desirable to provide a complementary process that integrates an NPN device, such as a SiGe NPN device, and a vertical PNP device. However, integrating a vertical PNP device and an NPN device, such as a SiGe NPN device, in the complementary bipolar process undesirably increases processing complexity and manufacturing cost.
Thus, there is a need in the art for a complementary bipolar process for effectively integrating a vertical PNP device and an NPN device, such as a SiGe NPN device, without undesirably increasing processing complexity and manufacturing cost.